Local heating by irradiation of a laser beam causes a phase change between an amorphous phase and a crystal phase of a thin film formed from a chalcogen material or the like on a substrate. The phases have different optical constants (a refractive index n and an extinction k) and such difference is caused by difference of the irradiation conditions. This is widely known and development and commercialization of a so-called phase-change type optical information recording medium are now actively advanced.
Applying the laser beam to the phase-change type optical information recording medium is made by applying the laser beam to an information track. The laser beam is applied while a laser power is modulated, depending on the information signals, between at least two power levels, one being a recording level and the other being a erase level. This makes it possible to erase existing signals from and record new signals on the medium at the same time.
In the optical information recording medium, protective layers, as layers other than the recording layer, are provided with the recording medium therebetween in a thickness direction. In other words, the protective layers are formed as a layer which is located nearer to the incoming laser beam (lower side), and a layer which is located further from the incoming laser than the recording layer (upper side). The protective layer is provided for preventing the evaporation of the recording layer and the thermal deformation of the substrate which are subjected to repetitive recording, and enhancing an optical absorbance and an optical change of the recording layer by optical interference effect. The protective layer is generally made from a dielectric material which has excellent heat resistance. Further, a reflective layer formed from a metal or an alloy material is generally provided in order to use the incoming light efficiently and to improve a cooling speed of the recording layer so that the recording layer is easily made amorphous. The reflective layer is formed at a position further from the incoming laser beam than the recording layer, that is, a position where it reflects the laser beam which has passed through the recording layer.
It is proposed that an interface layer is provided between the recording layer and the dielectric layer. The interface layer serves to promote the crystallization of the recording layer so that the erase characteristic is improved, and to prevent the mutualdiffusion of atoms or molecules between the recording layer and the dielectric protective layer so that the durability against the repetitive recording is improved. It is preferable that the interface layer is not liable to exfoliate from the recording layer and has environment reliability such that corrosion does not occur.
Further, it is proposed that a layer of a material which has a high refractive index and absorbs the light moderately is provided between the upper dielectric layer and the reflective layer. The purposes of providing this layer are as follows:    i) A ratio of the optical absorbance of the recording layer in crystal phase and the optical absorbance of the recording layer in amorphous phase is adjusted to suppress deformation of the mark shape upon overwriting, whereby the erase ratio is improved; and    ii) Difference in reflectivity between the recording layer in crystal phase and the recording in amorphous phase is made large to increase a C/N ratio.
In order that an information volume stored in one optical information recording medium, it is necessary to optimize an optical system for high-density recording. Basically, it is necessary to reduce a spot diameter of the laser beam by shortening a wavelength of the laser beam or increasing a numerical aperture of an objective lens which focuses the beam. The main stream of high-capacity recording mediums is one which requires the use of the optical system of a wavelength of 660 nm/a numerical aperture of about 0.6, which is represented by a recordable DVD (Digital Versatile Disc). Further, a recordable BD (Blu-ray Disc) has been commercialized, which requires the use of the optical system of a wavelength of 400 nm/a numerical aperture of about 0.85. A blue laser diode is employed for the recording and the reproduction of this medium.
In addition, a recording medium of multi-layer construction (which is also called as “multi-layer recording medium”) is also proposed, wherein a plurality of layers on or from which information is recorded or reproduced are stacked. The recording and the reproduction is made, on the information layer(s) which is further from a laser source in such a multi-layer recording medium, by using the laser beam which is damped since the information layer(s) nearer to the laser source absorbs the light. For this reason, there is a problem of decrease in sensitivity when recording, and there is a problem of reduction in reflectivity and amplitude when reproducing. Therefore, it is necessary to design the information layer(s) nearer to the laser source such that it has a high transmittance, and the information layer(s) which is further from the laser source such that it has a high reflectance, a large difference in reflectance, and a high sensitivity. This enables sufficient recording and reproduction characteristics with a limited laser power.
In the optical information recording medium, it is also important to increase a recording speed for processing a large volume of data for a short period of time while, as described above, it is important to increase the recording density. Among the mediums, a recordable DVD adapted to 5× recording includes a medium which is adapted to a wide range of linear velocity covering from a low linear velocity (linear velocity 8.2 m/s; 2× recording) to a a high linear velocity (liner velocity 20.5 m/s; 5× recording). The ratio of these linear velocities is 2.5 and therefore this medium is adapted to a very wide range of linear velocity.
The crystallization speed of the recording layer is required to be increased in order to adapt to the high-speed recording. A known technique of increasing the crystallization speed is, for example, changing a representative recording material Ge—Sb—Te, in particular, a composition around GeTe—Sb2Te3 (a composition which is positioned near a GeTe—Sb2Te3 line in a constitution diagram of a Ge—Sb—Te three-component system) to another composition around GeTe—Bi2Te3 by substituting Sb with Bi. Further, a technique of substituting a part of Sb with Bi (for examples, Patent Literatures 1 and 2)
Patent Literature 1: WO00/54982
Patent Literature 2: Unexamined Japanese Patent (Kokai) Publication No. 2004-311011